Interpolation of plotted points between sample values

ABSTRACT

A video display for digitized video data interpolates values between time samples, such as luminance over a horizontal line or over an average of all the horizontal lines in a video signal, so as to produce a digitally plotted test display resembling the continuous line display of an oscilloscope. A digital impulse response filter has stored coefficients that contribute as factors to the values of interpolated data points to fill the line display between sample values. Instead of changing the coefficients to produce variations interpolating the output between sample values, the coefficients are held constant and a delay factor is varied to alter the extent to which the respective coefficients and sample values interact and contribute to the interpolated values.

FIELD OF THE INVENTION

The invention relates to apparatus and methods for generating a graphicplot of digitally generated sample values, including inferring andplotting values for interpolated points between discrete time-sampledpoints to produce a more continuous representation of the plot.

The interpolated points can be made closely adjacent, resembling asubstantially continuous line. According to an inventive aspect,interpolated values are generated by repetitively selecting differentdelay values when applying a set of correlation factors to a filter thatproduces the interpolated values for the plot, for example a plot ofamplitude versus time in a display of video signal levels. Thistechnique avoids or minimizes the need to apply new factors to thefilter.

PRIOR ART

It is sometimes necessary to infer or interpolate values between valuesthat are known for a particular function to be plotted. In a plot oftime samples, for example, interpolated values may be inferred for avariable at times between the discrete sample times at which a valueactually is measured. The interpolation might be done for, variousreasons. An example is generation of a continuous line plot with theappearance of an oscilloscope display, from input data limited to thesampled values of a particular signal parameter at incrementally spaceddiscrete times, that otherwise would have to be plotted as spaced dotsor as a digitized stairstep pattern.

In video production test apparatus, analog plots of voltage versus timehave been used conventionally to display parameter values. Standardcolor bars and other test pattern signals produce familiar waveformshapes that are useful in test applications. An example is a plot ofluminance over a time period corresponding to a horizontal scan line,for a standard test pattern image of vertical color bars. Anoscilloscope sweep trace triggered according to the horizontal sync canplot time on a horizontal axis and a voltage representing luminance on avertical axis. (Other plots and/or variables of course are possible.) Ifthe scan is repeated while the video data is reasonably static, forexample while displaying a stationary color bar test pattern, a displayis produced that is useful for diagnostic purposes. Even if the videodata changes or if the scan is intermittently triggered, the persistenceof the oscilloscope phosphors can be sufficient to provide a displayedline. A storage oscilloscope can hold a displayed line for even a singlescan.

In time-sampled digital video data, instead of a continuously varyingvariable value, the video data is a succession of instantaneous samplevalues measured at regularly spaced times. A plot of the actual data,i.e., the successive sample values, consists of discrete points or dotsthat are regularly spaced at time increments corresponding to thesampling frequency. If the value of a sample is construed to persistuntil the next sample (which is a form of interpolation), the plotconsists of discrete segments at amplitudes that vary stepwise from onesegment to the next.

Interpolation techniques provide values during the time periods betweenthe discrete sampling points. Advantageously, the interpolated valuesare chosen to approximate how the variable value might be expectedactually to change between samples, insofar as possible.

It is possible to convert time-spaced digital samples back into ananalog signal wherein the parameter value is represented by acontinuously varying level of voltage or current. This is accomplishedby applying the samples to a digital-to-analog converter, passing theoutput of the D/A converter through a low-pass filter to replace astepwise or similarly varying signal with a smoother line, and finallyplotting the level of the output of the low-pass filter versus timeusing an oscilloscope-like display device. An analog version of anoscilloscope display device has beam deflection amplifiers that producea horizontal time sweep and a vertical position of the beam that varieswith the analog level.

If the device used for plotting the output is a digital displaygenerating device as opposed to an analog oscilloscope with X and Yamplifiers, a processor typically composes a display field of pixelswith contrasting brightness and/or hue. The processor generates a plotrepresenting parameter value versus time, by mapping out a line ofcontrasting pixels in a display memory to correspond to the image thatwould be produced by a swept beam in an oscilloscope or the like. Thedata in the display memory is used to produce a plot of amplitude versustime, but without involving corresponding beam deflection as in anoscilloscope. The plot is produced by mathematical scaling and dataprocessing techniques.

In the example of converting samples to analog through a low-pass filterfor interpolating values between samples, a digital display device mightbe used to produce display data by manipulating data a display memory.In such an example, it may be advantageous to convert the analog levelfrom the low-pass filter back again to digital, and to sample at a ratehigher than the original sample rate to produce interpolated values forprocessing by the display generator. It would be desirable to producethe data mathematically, without the complexity of converting back andforth between digital and analog to convert the original samples tolow-pass analog and then back to digital at a higher sampling rate.

Converting the data from a stepwise varying set of time samples into asubstantially continuously varying set of values for plotting requires amathematical function that resembles a low-pass filter. The functionneeds to produce more sample values for filling in the time span of aplot than are available from the input samples alone.

U.S. Pat. No. 6,493,024—Hartley et al. discloses using a digital filterto provide sample values at an effectively higher rate than the inputsampling rate, the values falling along a curve that can be smoothed byuse of filter correlation factors (e.g., having a low pass transferfunction), wherein additional values are interpolated between the knownsample values at a rate higher than the sampling rate. This concept maybe described as “oversampling” because the samples at the outputcorrespond to a shorter sampling time period (a higher sampling rate).The technique relies on applying a mathematical function to the sampledata values to generate virtual time sample values, and not on actualperiodic sampling or similar steps. The digital filter and associatedprocessor needs to operate at a sufficient rate to keep up with theinput samples and to produce the mapped data in the display memory. Thedigital filter need not be synchronous with a sampling clock or thelike.

In Hartley, a digital filter is employed, having a pattern or successionof several numeric factors applied to a stream of sample values, eachfactor producing from the sample values a new resulting value as thedata passes through the filter. The resulting values from the factorsfill in the plotted values to produce a substantially continuous plotfrom the discrete samples.

The digital filter in Hartley is a transversal finite impulse response(FIR) filter. The factors used are coefficients derived from a sin(x)/xlow pass impulse response curve. In one embodiment, the filter has fivestages or sets of coefficients. The mathematical products of thecoefficients times the sample values at each stage, are added todetermine plotted values. The coefficients define mathematicalcontributions of each input sample value over a span of values in theoutput.

In order to fill out the oversampled plotted values and to form arelatively continuous line in the plot of values in the output, thecoefficients applied to the digital filter in Hartley are varied. Thecoefficients are changed for each successive line of sample data,thereby producing slightly different output values. The variation can bean incremental displacement over time of the sin(x)/x pattern ofcoefficient values relative to the samples to which the factors areapplied. This smoothes the plot over the sampled values and also fillsin or interpolates portions of the plot between the resulting outputvalues.

The Hartley digital filter can be used to process one horizontal line ofvideo or can process an entire field of video wherein all the lines ofsample data in the field contribute to one plotted curve representing asingle line. When used for one line, the line sample data is recycled(used over and over again). Hartley changes the coefficients used foreach horizontal line or for each repetition of a given line by applyinga new set of coefficients to the filter for each line or repetition of aline. That is, the same filter configuration is used and thecoefficients are changed by an incremental phase difference. For eachline between the first and last lines of the video field, or for eachrepetition of a predetermined number of iterations of the same videoline, the sin(x)/x curve from which the coefficients are derived isphase shifted in discrete intervals between −180° and +180°.

In the Hartley technique, the coefficients used by the digital filterneed to be changed between iterations (changed from one horizontal lineto the next, or changed for each repetition of a single horizontal linethat is being recycled). Changing the coefficients might possiblyinvolve composing a set of coefficient values for use during eachiteration, or possibly by modifying the addressing of stored operandsused in successive computations. In any case, a point value iscalculated for each incrementally different phase or time and for eachline or iteration of a repeated line, thereby producing sample pointsthat are more numerous than the original sample values, and fill in thezones between samples. If the interpolated additional points aresufficiently numerous, the plot that resembles a continuous line.

These arrangements are complex from a standpoint of circuit elements,computational requirements and/or processing time. It would beadvantageous if a digital filter could use predetermined coefficientsrather than varying them, so that a smoothed oversampled plot can beobtained without the need to re-compute, reload or reselect coefficientsfor each line. However, it would seem if preloaded coefficients were tobe provided in Hartley, a large array of coefficients would be needed torepresent a different and discrete set for the sampled points in thehorizontal lines of a video field.

SUMMARY OF THE INVENTION

According to the present invention, a different technique is providedfor interpolating values to be used in graphically plotting a sampledvideo parameter as a line of changing amplitude values over time.According to one inventive aspect, a digital filter with predeterminedstatic coefficient values is subjected to a variable delay so as toproduce incrementally different values used as interpolated valuesbetween sample points. By using a variable delay technique withpredetermined coefficients, it is not necessary to employ changingcoefficients or to alter or shift the operands used in generatinginterpolated values. Moreover, according to another aspect, the extentof the variable delay can be randomized over a range of delays or variedaccording to a preset function.

An object of interpolation in this context is to generate fromtime-spaced digital samples a presentation that appears to be a plottedline resembling an oscilloscope trace. A digitally produced plot in apixel field, with or without interpolated values, comprises a line ofdiscrete pixel values. Thus the interpolated output values in a sensestill comprise discontinuous sample values after the interpolation. Theinterpolated values are more dense in the plot than the input samples.By use of the interpolation technique described herein, the outputplotted values can be made much more numerous than the time spacedsamples that formed the input data. The plotted values can so numerousthat the contrasting pixels that define a plotted trace line in acomposed pixel field representing the time plot are substantiallyadjacent to one another along the time axis, i.e., provide a line thatis digitized to a time resolution less than or equal to one pixelposition per interpolated value. An interpolated value plot also can begenerated by the inventive technique wherein the plot is scaled so thatthe interpolated values are more or less dense than the pixels.

According to a further inventive aspect, a set of fixed digital filtercoefficients is employed in a Farrow configuration having plural stages.Each stage has a set of coefficients and the coefficients in therespective stages can differ incrementally by randomized increments orcan differ according to a function. Each stage can carry fixed factorsaccording to a low pass finite impulse response filter pattern ofcoefficient values, such as a sin(x)/x pattern, wherein a parameter ofthe coefficient values such as the amplitude, period, relative phase orthe like differs from stage to stage.

A variable digital delay element selective alters the delay input to theinventive filter. This tends to vary the resulting interpolated valuesincrementally, in a manner that resembles a low pass function and alsofills in the values between time-spaced input samples. The filter stagesare coupled to sum and generate data output values that are oversampled,i.e., more numerous than the time-spaced input samples. By changing thevariable delay of the filter according to one or more sequences that canbe regular or irregular, the filter produces oversampled values in asubstantially more continuous progression than the input samples,preferably sufficient, in view of the pixel resolution of the outputdevice, to appear as a continuous line, but at least being morecontinuous than the input samples as similarly plotted.

The device is particularly useful for plotting points corresponding tosamples in a digitally generated display of video parameters such as thevalue of luminance over a horizontal line or over an entire video field.The device also can be applied to plotting other parameters in two axeswherein a series of spaced points are known and it is desirable to infera value on one axis for a substantially more numerous set of values onthe other axis. In connection with time sampled data, one axispreferably is a time axis.

Accordingly, the inventive video display for digitized video datainterpolates values between time samples, such as luminance over ahorizontal line or over an average of all the horizontal lines in avideo signal, so as to produce a digitally plotted test displayresembling the continuous line display of an oscilloscope. A digitalimpulse response filter has stored coefficients that contribute asfactors to the values of interpolated data points to fill the linedisplay between sample values. Instead of changing the coefficients toproduce variations interpolating the output between sample values, thecoefficients are held constant over at least a predetermined period suchas one or more video fields. Instead of varying the coefficients duringthat period, the coefficients remains and a delay factor is varied toalter the extent to which the respective coefficients and sample valuesinteract and contribute to the interpolated values. This arrangement ischaracterized, for example, in a Farrow filter arrangement where thedelay factor can be provided as an input variable to the filter. Thedelay can be varied randomly or pseudorandomly, or according to a schemesuch as an incremental progression of successive delays. The successionof different delays can be generated or simply read out from a set ofstored values.

Additional objects and aspects of the invention will become apparentfrom the following discussion of examples.

BRIEF DESCRIPTION OF THE DRAWINGS

A number of additional objects and aspects are apparent from theappended description and the associated illustrations of preferredembodiments, wherein:

FIG. 1 is a schematic block illustration of the method according to theinvention, showing the picture and plot details associated with sampleddata inputs as applied to video data.

FIG. 2 is a block diagram illustration of a digital filter applicable tothe invention, in this case having a Farrow configuration whereinpredetermined impulse response coefficients are provided and variablyapplied according to a controllable delay.

FIG. 3 is a plot representing one possible succession of delays, namelya randomized succession, using a given set of coefficients and a variedsuccession of delay values.

FIG. 4 is a plot substantially as in FIG. 3, except that the delays aresuccessively advanced by an incremental time or phase value.

FIG. 5 is a plot labeled “prior art,” showing for comparison the knowntechnique of changing a coefficient set repetitively.

DETAILED DESCRIPTION

The video test apparatus and method of the invention are functionallyoutlined in FIG. 1. The arrangement produces a display plot 22 thatresembles a display that one might obtain from an analog oscilloscopecoupled to a composite analog video signal, but the inventivearrangement operates on digital samples. A YCbCr source 26 of sampledata is shown in FIG. 1. By way of example, the sample data 26 isillustrated as representing a color bar test pattern 31 as the content,namely with vertical bars 33 that vary in luminance, generally having areduction in luminance were a given band 35 abuts the adjacent bands.

The sample data 26 could represent any sort of data that contains twovariables, such as an amplitude variable and a time variable. Luminanceis intended as a nonlimiting example and is useful because the variationin luminance across the transitions between successive bands produces adistinct drop in the plotted sample values of luminance over time. Asshown in FIG. 1, the plotted samples are simply points. However, byapplying a digital filter having finite impulse response (FIR)coefficients C₀ to C_(n), a set of interpolated values are filled inbetween the samples and thereby produce a line plot 22.

The source 26 of time sampled input data values representing a videoparameter at incremental sample times can be any source of a serialstream of digital data samples, e.g., from a broadcast or playbackapparatus, or as the output of an analog to digital converter (notshown).

As shown in FIG. 2, the digital filter shifts the values X of thesamples through registers while applying the coefficients C₀ to C_(n) tothe samples X being applied to the filter and summing the contributionof the coefficients and the sample values to produce an output value Z.The sample values X are multiplied repetitively or iteratively by thecoefficients C_(n) to obtain output values Z that are more numerous thanthe input values X. The coefficients C₀ to C_(n) represent an impulseresponse, and can in particular represent, for example, a sin(x)/xfunction as shown in FIGS. 3 and 4. By providing more numerous values,the filter can produce values that are interpolated between the plottedsample values or the filtered plotted sample values.

Assuming that a same sample value is applied to the digital filter overand over, the result would be the same value for output Z each time,i.e., a stair step function instead of a set of interpolated values thatvary meaningfully with changes in the input samples X. According to theprior art, such as U.S. Pat. No. 6,493,024—Hartley et al., the values inof the coefficients C₀ to C_(n) are changed for each repetition, thisbeing generally depicted in FIG. 5 as a set of different transferfunction shapes. The Hartley procedure produces interpolated values thatare incrementally different and can be used to interpolate pointsbetween the otherwise unvarying output value or values Z that would beproduced from a given input value or succession of input values X. Thedrawback of Hartley is that changing the coefficients, whether bycalculation or by reading from a memory, requires either complicatedcircuitry or time consuming data processing steps.

According to an aspect of the present invention, interpolated outputvalues that are incrementally different are provided without changingthe coefficients C₀ to C_(n) for each iteration. Instead, a delay inputα is provided and changed to alter the timing relationships by which aset array of coefficients C₀ to C_(n) contribute to each output value Z.In the illustration in FIG. 2, a delay factor α is applied to the filterto adjust the delay by which the factors C₀ to C_(n) are applied intheir contribution to the output value Z and this delay factor is variedby a control register 72.

The delay value α can be varied according to a sequence, changing witheach iteration to a new value and repeating periodically. This effectiveproduces a slightly different relationship between the input values andthe output values, in a way that is less complex than varying thecoefficients or factors C₀ to C_(n). A greater or smaller delay factorproduces a different FIR digital filter output for each iteration and/orinterpolated output value. Instead of changing the transfer functioncoefficients, the result is achieved by varying the delay value appliedto the digital Farrow filter.

In a Farrow filter as shown schematically in FIG. 2, the filter hasplural coefficients C₀ to C_(n) applied successively to the input datavalues in computation stages 51, 52, 53, etc. At each of the computationstages, a coefficient C₀ to C_(n) produces from a corresponding inputdata value Z a product. The products for said plural coefficients andsets are iteratively shifted toward an output 60 and contribute to theoutput values Z. The output values Z are a filtered version of the inputvalues X, and include interpolated additional values between the inputvalues X.

As shown in FIG. 2, the output values Z are coupled to a displaygenerator 70. This display generator can comprise a processor that alsodetermines the changing value of delay α via a control register 72 thatstores one or more delay values.

The display generator 70 plots the stream of output values Z over a timescale, namely by altering the appearance of pixels in a pixel datamemory to be employed by a display driver (not shown) to produce adisplay that can be presented on a monitor. Advantageously, the displayis generated as a VGA, SVGA or other standard display field that can bedisplayed on generic or proprietary monitors as desired.

The coefficients C₀ to C_(n) of the inventive digital filter areconstant, at least during a phase of operation of the interpolationapparatus during which multiple input values are processed. It ispossible also to change the coefficients or to change the coefficientsadaptively or by user selection, as well as changing the delay asdescribed. But according to the invention it is not necessary to changethe coefficients because meaningful interpolated values can be producedfrom a given set of coefficients C₀ to C_(n) by varying the delay factoras described. In a simple application as shown in FIG. 2, a single delayvalue α can be applied between each successive stage. This tends toalter the transfer function as shown in FIG. 3. Other specificvariations are also possible, such as applying random delays to therespective stages (such as a random number of clock cycle delays up to agiven maximum). Nevertheless, provided a delay input is coupled to thedigital filter, and varied for altering the application of thecoefficients to the input data values during a phase wherein thecoefficients remain constant, the result is interpolated values at theoutput Z for use by the display generator 70, to plot as output datavalues as a function of time.

The invention is primarily applicable to application of a Farrow typedigital FIR filter to a video test apparatus wherein it is desirable toplot a serial stream of video parameter values, i.e., video datasamples. The generator is enabled by the interpolation as described toplot a substantially continuous line of output data values in a displayplot that appears much the same as an oscilloscope trace and representsat least one horizontal line in a video picture. In connection with atest pattern display, a single line plot can represent the compositeoutput produce by repetitively filtering each successive horizontal linein a picture field or frame as the source of iterative input data thatat least nominally remains the same from line to line.

The coefficients of the digital filter represent low pass filtercoefficient arrays. Unlike the prior art of FIG. 4, wherein the filterapplies the same transfer function with a phase delay, the inventivefilter as shown in FIG. 3 can alter the transfer function of the lowpass filter to produce interpolated values.

The coefficients of the digital filter represent low pass filtercoefficient arrays having different attributes. The attribute canencompass different values of at least one of amplitude, period andphase. However, the variability in the values obtained for therespective interpolated points is not produced by regenerating orshifting among different sets of coefficients. Instead, the variabilityarises by applying the factors in a Farrow filter arrangement as in FIG.2, that define different delays for the application of each coefficientin each iteration or each new horizontal line of video data. Thisproduces a set of different results by varying the application of thecoefficient array without the need to change the coefficient values.This difference is shown in an exemplary arrangements in FIGS. 3 and 4,which show alternatives ways to change the delay input factor applied tothe coefficient stages. This technique is distinct from varyingcoefficient value patterns, or perhaps successively varying thecoefficients for different phases or transfer function envelopes, shownin FIG. 5 and labeled as prior art (e.g., as in Hartley). The inventivetechnique is computationally uncomplicated and requires minimalassociated circuitry.

In the inventive technique, at least one of the filter coefficientarrays can represent a sin(x)/x transfer function. This is also true inHartley. The inventive technique specifically employs a Farrow filter asopposed to a digital FIR with iteratively re-computed or iterativelyre-written coefficients as in the Hartley technique.

The control 72 coupled to the delay input α of the digital filter variesthe manner of application of the fixed coefficients to the data values.Repeating a given succession of input data samples, for example byrepetitively processing successive horizontal lines in a color test barpattern or repetitively processing one or more individual horizontallines that may be selected for display, produces a succession ofslightly different output values, due to time variation of the delayinput by the control. In this way and as shown in FIG. 1, the discretedots of time-spaced plotted sample values are replaced with asubstantially continuous line of closely spaced interpolated values, thedata values between the plotted samples being inferred using filtercoefficients that according to different delay values.

The control can vary the delay input in any of a number of differentways that provide a variation in the data applied for each interpolationpoint. The delay can be varied according to one or more of a timechanging function of incrementally different delays between endpointsdefining maximum and minimum delays, or by cycling through a successionof stored delay values. Another alternative is to select delay valuesaccording to a random or pseudo random succession.

According to the foregoing description, the interpolated values aredetermined for introducing new points between the samples as shown inFIG. 1. The actual generation of the display plot is a function of thedisplay generator 70, shown coupled to the output of the digital filterin FIG. 2. The digital filter can be a dedicated circuit or a programmedgate array device, or simply a function of a processor controlled by itsprogramming to make the necessary calculations. The display generatorlikewise can involve a separate device or a function of a processorhaving this and other functions as well. The display generator cancomprise a conventional pixel memory from which a display driver willproduce a signal for controlling a display device. Preferably, thedisplay generator has a stored background image that would produce ablank graphic plot, and is programmed to populate the graphic plot byaltering the luminance, color or other contrast producing attributes ofpixels corresponding to the spatial points at which the plotted line isto appear. Thus, the output values are mapped to addresses in the pixelmemory so as to produce a plot of amplitude (or other quantifiedparameter) versus time. The plot can be generated repetitively or onlyselectively generated. Various options can be selected includingchanging the displayed parameter, changing the time or amplitudescaling, selecting the portion of the signal to be processed (e.g., onehorizontal line or several or all of the horizontal lines, etc.).

Preferably, the data plotted to the pixel memory by the displayprocessor is mapped on an incremental time scale by which at least oneof said output values is plotted to each of plural positions along atime axis. More preferably, every pixel position along the time axis canhave at least one point plotted to represent a value along the amplitudeaxis. As a result, the plot appears to be a continuous line at theresolution of the time axis of pixel positions, even though the inputsamples may have been less numerous than the number of pixel time axispositions available to the display device.

From an apparatus standpoint, the invention concerns an interpolatingdevice for operating on samples from an input receiving successivediscrete input data values of a parameter defined by at least twovariables. Preferably the at least two variables include a direct valueor a derivable value as a function of time (Y=f(t)) and are presented byrepetitive time samples.

A digital filter (FIG. 2) produces from each of the input data values(FIG. 1: video data samples), a set of plural output data values(interpolated values). The output values are more numerous than theinput values and the output values interpolate between the input values.According to an inventive aspect, the digital filter has a plurality ofcoefficients (FIG. 2: C₀-C_(n)) applied to the input data values throughat least one computation stage, wherein each of said coefficientsproduces from each said input data value α product contributing to atleast one of the data values of the output set. The coefficients of thedigital filter remain constant during at least a phase of operation ofthe interpolation apparatus during which plural input values areprocessed. A delay control is coupled to the digital filter, the delaycontrol altering the application of the coefficients to the input datavalues during the phase wherein the coefficients remain constant. Asdiscussed, the preferred digital filter comprises a Farrow type filterwith the input samples applied to factors representing a coefficientpattern, subject to the delay.

From a method standpoint, processing the video parameter value sampledover time according to the invention includes applying to a digitalfilter a source of time sampled input data values representing the videoparameter at incremental sample times; computing with the digital filtera set of plural output data values for each of the input values, therebyinterpolating values for times between the sample times, wherein thedigital filter has plural sets of stored coefficients appliedsuccessively to the input data values through at least one computationstage, each coefficient in the plural sets producing from acorresponding input data value a product, the products for said pluralcoefficients and sets contributing to the output values including saidinterpolated additional values according to a polynomial functionembodied by the digital filter; wherein the coefficients of the digitalfilter are held constant during at least a phase of operation of theinterpolation apparatus during which plural input values are processed;providing a delay control to the digital filter, for altering theapplication of the coefficients to the input data values during thephase wherein the coefficients remain constant, thereby causingvariations in the output values for a given set of input values due tothe delay control.

The video parameter is a video data sample and the output values areplotted to points on a display. Preferably the plot represents at leastone horizontal line in a picture. Alternatively, and preferably subjectto selection of an operational mode, the output data values that areplotted can represent multiple horizontal lines in the picture.

It is an aspect of the invention that instead of re-computing orre-configuring a set of coefficients (e.g., at incrementally differentphase positions), the coefficients of the invention are predetermined,i.e., permanently stored. The coefficients can represent a low passfilter coefficient array, such as a sin(x)/x pattern or the like, storedas the coefficients of the digital filter. However, it is also possibleto permit a selection among different arrays or patterns according toprogrammed selection or user input, for choosing low pass filtercoefficient arrays so as to include different attributes for at leastone of amplitude, period and/or phase. This choice can involvecomputation or randomization of values, but by embodying the digitalfilter as a Farrow filter or other filter having a varying delay, theinvention permits a given set of values to generate interpolated pointsthat are more numerous and are incrementally different from one anotherand from the time spaced sample points, such that the samples can berepresented as a substantially continuous plotted line of points.

The invention having been disclosed in connection with the foregoingpreferred arrangements, variations will now be apparent, and should beconsidered encompassed within the scope and spirit of the invention.

1. A video test apparatus, comprising: a source of time sampled inputdata values representing a video parameter at incremental sample times;a digital filter operable to produce from each of the input data valuesa set of plural output data values, wherein the output values compriseinterpolated additional values of the parameter between the sampletimes; wherein the digital filter has plural sets of coefficientsapplied to the input data values through at least one computation stage,wherein each coefficient in the plural sets produces from acorresponding input data value a product, the products for said pluralcoefficients and sets contributing to the output values including saidinterpolated additional values; wherein the coefficients of the digitalfilter are constant during at least a phase of operation of theinterpolation apparatus during which plural input values are processed;further comprising a delay control coupled to the digital filter, thedelay control altering the application of the coefficients to the inputdata values during the phase wherein the coefficients remain constant; adisplay generator operable to plot the output data values as a functionof time.
 2. The video test apparatus of claim 1, wherein the videoparameter is a video data sample and the display generator plots theoutput data values as a representation of at least one horizontal linein a picture.
 3. The video test apparatus of claim 2, wherein the outputdata values are a representation of multiple horizontal lines in thepicture.
 4. The video test apparatus of claim 1, wherein thecoefficients of the digital filter represent low pass filter coefficientarrays.
 5. The video test apparatus of claim 4, wherein the coefficientsof the digital filter represent low pass filter coefficient arrayshaving different attributes in at least one of amplitude, period andphase.
 6. The video test apparatus of claim 5, wherein at least one ofthe filter coefficient arrays represents a sin(x)/x transfer function.7. The video test apparatus of claim 6, wherein the digital filtercomprises a Farrow configuration of said coefficients and saidcomputation stage.
 8. The video test apparatus of claim 7, furthercomprising a control coupled to a delay input of the digital filter forvarying said application of the coefficients to the data values, wherebyrepeating a given succession of input data samples produces a differentsuccession of output values due to time variation of the delay input bythe control.
 9. The video test apparatus of claim 7, wherein the controlapplies to the delay input one of a time changing function, a successionof stored values and an at least partly randomized set of values. 10.The video test apparatus of claim 1, wherein the display generatorcomprises a pixel memory and wherein the output values are mapped toaddresses in the pixel memory.
 11. The video test apparatus of claim 10,wherein the addresses in the pixel memory are mapped on an incrementaltime scale by which at least one of said output values is plotted toeach of plural positions along a time axis.
 12. An interpolatingapparatus, comprising: an input for receiving successive discrete inputdata values of a parameter defined by at least two variables; a digitalfilter operable to produce from each of the input data values a set ofplural output data values, wherein the output values are more numerousthan the input values and the output values interpolate between theinput values; wherein the digital filter has a plurality of coefficientsapplied to the input data values through at least one computation stage,wherein each of said coefficients produces from each said input datavalue a product contributing to at least one of the data values of theoutput set; wherein the coefficients of the digital filter remainconstant during at least a phase of operation of the interpolationapparatus during which plural input values are processed; and, furthercomprising a delay control coupled to the digital filter, the delaycontrol altering the application of the coefficients to the input datavalues during the phase wherein the coefficients remain constant.
 13. Amethod for processing a video parameter value sampled over time,comprising: applying to a digital filter a source of time sampled inputdata values representing the video parameter at incremental sampletimes; computing with the digital filter a set of plural output datavalues for each of the input values, thereby interpolating values fortimes between the sample times, wherein the digital filter has pluralsets of stored coefficients applied to the input data values through atleast one computation stage, each coefficient in the plural setsproducing from a corresponding input data value a product, the productsfor said plural coefficients and sets contributing to the output valuesincluding said interpolated additional values according to a polynomialfunction embodied by the digital filter; wherein the coefficients of thedigital filter are held constant during at least a phase of operation ofthe interpolation apparatus during which plural input values areprocessed; providing a delay control to the digital filter, for alteringthe application of the coefficients to the input data values during thephase wherein the coefficients remain constant, thereby causingvariations in the output values for a given set of input values due tothe delay control.
 14. The method of claim 13, wherein the videoparameter is a video data sample and further comprising plotting theoutput values on a display for representing at least one horizontal linein a picture.
 15. The method of claim 14, wherein the output data valuesare a representation of multiple horizontal lines in the picture. 16.The method of claim 13, comprising permanently storing low pass filtercoefficient arrays as the coefficients in the digital filter.
 17. Themethod of claim 16, further comprising choosing the low pass filtercoefficient arrays so as to include different attributes for at leastone of amplitude; period and phase.
 18. The method of claim 13,comprising configuring at least part of the digital filter as a Farrowfilter.
 19. The method of claim 13, comprising applying to the delaycontrol at least one of a time changing function, a succession of storedvalues and an at least partly randomized set of values.